Need to add a README with cross-references, scope, etc.

[EricCousineau-TRI]

No README currently. The new readme should have something like:

• A pointer to sdformat.org
• A pointer to the sdformat repository
• Development tips
  • How to preview branches
  • Contribution guidelines?
    • https://ignitionrobotics.org/docs/all/contributing (currently still catered towards BitBucket)
    • https://index.ros.org/doc/ros2/Contributing/Developer-Guide/#pull-requests

• For previewing branches, need to pass ?repo= as well as ?branch=.

[EricCousineau-TRI]

Add'ly, it would be nice if the repository description (a) at a minimum used SDFormat (not "SDF"), and (b) perhaps linked to sdformat.org as well.

[EricCousineau-TRI]

Contribution-related stuff discussed here: gazebosim/sdformat#243 (comment)

[EricCousineau-TRI]

See example of ?repo= usage: #3 (comment)

[EricCousineau-TRI]

FTR Just some meta-level related issues (on documentation, traceability, etc.):

• README: Should have cross-references between urdfdom_headers and urdfdom? ros/urdfdom_headers#63 - for the modeling format URDF (highly related to SDFormat)
• https://github.com/ros2/ros_core_documentation/issues/12
• redundancy: Relevance to index.ros.org? ros2/ros_core_documentation#13

Posting in the context of:
https://community.gazebosim.org/t/gsod-2020-ignition-robotics-on-boarding-guide/576